1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) exchange, and more particularly to a cell bandwidth control apparatus for performing bandwidth control for ATM cells sent from the ATM exchange onto the line side.
2. Description of the Related Art
In an ATM exchange, a plurality of line adapters are connected to an ATM switch. ATM cells are input to the ATM exchange by payloads incoming from lines operating at various speeds; generally, the internal circuitry of the ATM exchange accommodating the lines is operating at a speed different from the speeds of the lines, and its operating speed is higher than the operating speeds of the lines.
Data formatted, for example, to SONET (Synchronous Optical Network), DS3, or other specifications, is input from the line side to the line adapter. The line adapter removes the overhead (OH), header, etc. of the data, extracts ATM cells carried in the payload, and sends the ATM cells to the ATM switch after converting each cell into an ATM switching cell with an internal routing tag appended thereto. Conversely, for ATM cells input to the line adapter from the ATM switch, the line adapter removes the ATM switching tag from each ATM cell, and maps the cells into the payload applicable on the line side, and sends them out onto the line side after appending the overhead, header, etc. of SONET, DS3, etc.
In this way, since the overhead (OH), header, etc. of SONET, DS3, etc. are appended when outputting an ATM signal from the line adapter onto the line side, the effective bandwidth for an outgoing highway signal that can be carried in the payload on the line side, i.e., the combined signal of user cells, OAM (Operations, Administration, and Maintenance) information cells for higher layers, etc., is 149.76 Mbits/s.
When the bandwidth of 149.76 Mbits/s is fully used, and OAM cells, for example, are inserted in the remaining bandwidth of 5.16 Mbits/s (155.52 Mbits/s-149.76 Mbits/s=5.76 Mbits/s) at the ATM exchange, a problem arises in that the bandwidth increases correspondingly and data equivalent to the bandwidth of the inserted OAM cells is discarded at the line side regardless of the type of cell, whether a user cell or an OAM cell.